Pt/Co multilayers have been used for MO recording media. These materials have perpendicular magnetic anisotropy, large Kerr rotation at short wavelength and excellent environmental stability. However, these materials when deposited by conventional sputtering processes possess low coercivity. Attempts have been made to increase the coercivity by controlling the Co/Pt ratio, using seed layers of metals and dielectrics, and etching the substrate before depositing the multilayer (see Magnetooptic Recording Medium, European Patent Application 0304873 (1988)); Perpendicular Magnetic Recording Media, European Patent Application 0304927 (1989); U.S. Pat. No. 5,082,749; and "Dielectric Enhancement Layers for a Co/Pt Multilayer Magnetooptic Recording Medium", Carcia et al, Appl. Phys Lett. 58 (1991), pages 191-193. For high Co/Pt ratio, even though coercivity up to 1000 Oe can be obtained, the hysteresis loops are not square. For through substrate recording, the use of a metal seed layer is objectionable since it attenuates the Kerr rotation, decreases the figure of merit and decreases the recording sensitivity. Some of the dielectrics such as oxides and nitrides of Zn, In, Al, Ta, Mg, Si, Sn, Fe, Zr, Bi metals increase the coercivity but the thickness of these layers is usually recommended to be more than 100 nm for sufficient enhancement. This also increases the deposition time and lowers the production throughput. Further, the crystalline size of the material is directly proportional to the thickness of the layer. Large crystalline size in thick seed layers may give rise to the noise during the recording process.
Furthermore, most oxides and nitrides are difficult to deposit using simple non-reactive dc sputtering at high deposition rate. The low deposition rate and the large structure thickness require long deposition time, result in low production throughput, induce thermal damage to the sensitive substrate such as PC and gives rise to mechanical stresses causing deformation of the structure. In extreme cases cracking and delamination of the structure may occur.
In the above-identified U.S. patent application Ser. Nos. 076,604, an amorphous seed layer is used which has a thickness of less than 20 nm. In U.S. Pat. No. 5,407,755 a polycrystalline seed layer is disclosed. Both of these seed layer structures provide a number of advantages.